1. The Field of the Invention
The present invention relates generally to surgical devices and methods for supporting bone or other tissues and, more specifically, to surgical devices and methods for fusing adjacent vertebrae or other bones.
2. The Relevant Technology
The spinal column is made up of thirty-three vertebrae, each separated by an intervertebral disc. Each disc is slightly compressible, thereby allowing the vertebra above a disc to move relative to the vertebra below the disc. This unique design allows the spine to bend in many directions. The intervertebral discs absorb pounding and compressive forces throughout the lifetime of a person. Through disease, trauma, or normal wear, an intervertebral disc can become damaged or ruptured, thereby creating instability that leads to loss of function and excruciating pain. Such persons often turn to surgery to remove the damaged disc and fuse the corresponding adjacent vertebrae together.
During surgery, the damaged disc is removed and a spinal fusion implant is inserted to replace the damaged disc and restore the spacing between the vertebrae. The spinal implant typically has a thickness corresponding to the thickness of the disc being removed and has openings extending therethrough. To facilitate permanent fusion between vertebrae, the openings of the implant are typically packed with an osteogenic substance. The osteogenic substance promotes the rapid growth of a bony column between the vertebrae. Once the vertebrae are fused, the two adjacent vertebrae act as one, rigid vertebrae.
When first inserted, the osteogenic substance is not sufficiently strong to withstand the compressive forces applied by the vertebrae. Hence the need for the implant. The osteogenic substance promotes the bone growth between the vertebrae until the bone growth fuses the vertebrae together and can independently withstand the compressive forces applied by the vertebrae. This fusion process can take several months to complete.
Although the osteogenic substance is not initially strong enough to withstand the full compressive force that a healthy disc can handle, bone growth produced by the osteogenic substance is greatly benefited by the osteogenic substance being subject to a compression force when first implanted. That is, for the osteogenic substances to form the bony growth between the vertebrae, the osteogenic substance should be firmly compressed between the vertebrae to prevent the osteogenic substance from moving or sheering relative to the bone. If the osteogenic substance is not compressed firmly between the bone, sheering or movement can occur leading to only a partial fusing or even no fusing to occur. Under such situations, surgery is often required to remove the implant and repeat the procedure.
Although there are many different implants that have been used to fuse vertebrae together, conventional implants can suffer from a number of shortcomings. For example, to withstand the compressive force initially produced by the vertebrae, many conventional implants have been structurally reinforced to such an extent that they have substantially no or minimal compression during use. As a result of the rigid structure of the implant, the osteogenic substance housed within the implant is not properly compressed between the vertebrae to effectively produce the bone growth as discussed above. The lack of compression of the osteogenic substance as a result of the implant is referred to as stress shielding.
Furthermore, the structural reinforcing of many conventional implants has been designed such that it limits the number of openings formed on and extending through the implant. As a result, it can be difficult for the bone growth to extend through the implant so as to fuse the adjacent bone together.
Other implants permit flexing at portions of the implant but fail to permit flexing along the full length of the implant, thereby minimizing the effective use of the osteogenic substance. Still other implants accommodate compression or minimize the need for compression by being formed from multiple parts that enable expansion of the implant between the vertebrae. Expandable implants, however, are typically more expensive, requiring special insertion and expansion tools, and can increase the complexity and time for implanting. Expandable implants can also have a high risk of failure under compression.
Accordingly, what is needed in the art are improved bone fusion implants that are simple and easy to implant, that provide desirable compression along the full length thereof so as to optimize bone growth produced by an osteogenic substance, and that are sufficiently open to enhance bone growth through and around the implant.